It should be appreciated that in this disclosure the term “concrete volute pump” means a centrifugal pump that has a volute or scroll whose wall comprises concrete. Concrete volute pumps constitute an effective technical solution for pumping large quantities of water or other liquids at very high flow rates. Such pumps for use in large power plants may attain volume flow rates of 20 to 40 cubic meters of water per second or even more, for total heads of water of up to 35 meters or more.
This type of pump comprises a bladed rotor or impeller which acts on the liquid by making use of centrifugal force to accelerate it, and a collector, or volute, disposed around the impeller. The liquid to be pumped typically enters the pump axially via an inlet pipe of the pump coaxial with the impeller shaft, and the flow is discharged via the blades towards the periphery of the impeller and into the volute.
The volute is a fixed body, with increasing cross-section towards its outlet, in which progressive retardation of the liquid discharged from the centrifugal impeller converts the kinetic energy of the liquid to pressure. The volute channels the liquid to its outlet and reduces turbulence and velocity of the liquid.
Because of the high liquid pressures prevailing in concrete volute pumps and the asymmetry of the volute, a radial thrust, perpendicular to the impeller shaft, is exerted on the impeller. The radial thrust gives rise to deflection of the impeller shaft, which may give rise to contacts between the impeller and adjacent static components. Such contacts may result in a major deterioration in the condition of the equipment as well as a loss of sealing of the pump and reduced rate of delivery.
Another problem that may arise in a concrete volute pump is damage to the concrete caused by the very high flow rates of the liquid circulating in the volute.
There are various prior art solutions that enable one or other of these problems to be resolved.
Regarding the problem of the radial thrust of the liquid on the impeller, a method is known for using one or more hydrodynamic bearings or ball bearings designed to render the impeller shaft more rigid, such bearings typically being located halfway along the shaft. However, this solution adds to manufacturing cost and requires additional maintenance work.
A method is also known for using dual concrete volute pumps to reduce the radial thrust on the impeller. These pumps suffer from the disadvantage that they are expensive and provide only a low delivery rate.
Moreover, the above two solutions do not solve the problem of wear of the concrete under the action of the flow rate.
As regards the problem of wear of the concrete, a method is known for using metal shields in the areas of the volute where the fluid flow rate is at its highest. The implementation of this solution is a complex and onerous task and does not solve the problem of radial thrust exerted on the impeller.
The present invention is intended to eliminate or reduce these disadvantages. The invention proposes, in particular, a concrete volute pump that reduces the unevenness of the radial thrust exerted on the impeller, whilst limiting the wear of the concrete in a simple, economic manner.